1. Field of the Invention
The present invention relates in general to survey devices and in particular, to an apparatus and method for automatic locking ("autolocking") a total station to a target.
2. Prior Art
The art of surveying involves the determination of an unknown position from angular and distance measurements taken from one or more reference positions. A surveying device frequently used to measure distance and angles to a target is a total station. In operation, the user points the total station at a target held by a second user. Typically, the pointing process is achieved optically (e.g., by the user visually locating the target through a view finder type of apparatus). Robotic total stations have been developed to assist in locating the target. The robotic total stations include servo motors that allow the apparatus to be rotated so as to align its measuring units towards a reflective target. Once aligned, the reflective target may be used for measuring the angle to, and the distance from the surveying system. Examples of such robotic surveying systems include the Geodimeter System 600 manufactured by Geotronics AB of Sweden, and the TPS-System 1000 total station manufactured by Leica of Switzerland.
A robotic total station eliminates the need to have a user at the location of the total station. Instead, the user is at the target. With one system the user sights the total station visually through a view finder apparatus located on the target. Upon visually locating the total station, the user initiates an angular scanning search sequence at the total station so the total station can find and lock onto the target for more precise angular and distance measurements.
During the set-up of some robotic total stations, the user is prompted to provide the limits of a scanning window or search zone by specifying the elevational and azimuthal limits of the search. This is accomplished, for example, by marking the highest point of the scanning zone on the left hand corner of the viewfinder, and the lowest point of the scanning zone on the right hand corner of the viewfinder. The two specified points enable the robotic surveying system to set up the window within which the scanning sequence is performed. Once the scanning is set, the user can place the target at the location to be measured, sight the total station and remotely initiate an angular scanning sequence which the system is programmed to perform to precisely locate the target. During the scanning sequence, the total station transmits a signal. Reflection of the signal by the target will only occur when the total station is substantially aligned with the target. The reflected signal received by the total station is used in calculating the location of the target. More particularly, the stronger the received reflected signal, the more likely the total station is focused centrally on the target.
One problem with such conventional robotic total stations is that the target must first be identified or generally observed by the total station before the location station can locate the target. In addition, the angular scanning angles of the total station are limited so that the user may have to adjust the total station and reinitiate the process with the target. This is a very cumbersome task, especially if the total station is not within close physical proximity of the user, since the total station has to be manually reset. Furthermore, if the target is obstructed by the terrain or by buildings, the radio link provided by such conventional robotic total stations will also be obstructed, and attempts for locating the target will be unsuccessful. Moreover, since the target has to be visually located, and the signal from the surveying system must be reflected off the target, it is difficult to locate and to lock onto targets over long ranges.
Accordingly, there is a need in the technology for a method and apparatus for providing a search routine which facilitates accurate, robust and efficient location of a target using a robotic total station.